Typical refrigerator appliance cabinets consist of an outer metal cabinet, an inner plastic liner, typically ABS (acrylonitrile-butadiene-styrene) or HIPS (high impact polystyrene), and an insulating foam core, typically polyurethane foam. Blowing agents for the polyurethane foam are locked into the foam. Freon, a completely halogenated methane, fluorotrichloromethane, is presently employed commercially as the blowing agent. For environmental reasons, implemented by regulations, substitutes for Freon must be found. Proposed substitutes for Freon are halogenated hydrocarbons which contain at least one hydrogen atom.
Polyurethane blowing agents, such as Freon (CFC-11) and Freon substitutes, such as 2-fluoro-2,2-dichloroethane and 2,2-dichloro-1,1,1-trifluoroethane (HCFC 141b and HCFC 123, respectively), can cause liner blistering, catastrophic cracks, tiny cracks (crazing), and loss of impact properties (embrittlement), as well as stress whitening and/or dissolution. The blowing agents HCFC 141b and HCFC 123 appear to be more chemically aggressive than Freon (CFC-11) in attacking the liner. It is the common belief that blowing agent attack of the liner occurs on condensation of the blowing agent to liquid, which occurs on cooling. Cooling and condensation of the blowing agent does occur during shipping and storage. Shipping conditions are simulated during fabrication by cycling the appliance cabinet from hot to cold to cause evaporation and condensation of the blowing agent(s).
It is proposed to provide a plastic sheet structure to be thermoformed into a refrigeration liner that is resistant to chemical attack.
It is an object of the invention to provide a refrigeration appliance liner to be fabricated from a thermoformable, plastic sheet material exhibiting resistance to chemical attack (blistering, cracking, crazing, as mentioned above), by polyurethane foam blown with Freon (CFC-11) or potential Freon substitutes, including HCFC-123 and HCFC-141b, which are mentioned above.
It is an object of the invention to provide a refrigeration appliance liner to be fabricated from a thermoformable, plastic sheet material which retains a high level of toughness (impact properties) and strength (tensile properties), even at low temperatures (at 5.degree. F. or less).
It is another object of the invention to provide a liner made from a plastic sheet material that maintains processability similar to HIPS or ABS, including favorable extrusion conditions and similar thermoforming behavior.
It is another object of the invention to provide a liner made from a plastic sheet containing a layer of a special multi-functional blend that exhibits excellent chemical resistance to Freon or potential Freon substitutes, may additionally function as an adhesive layer between optional layers of HIPS (or ABS) and polyolefin, and finally acts as a compatibilizing agent when regrind plastic sheet scrap is recycled to virgin plastics resin being extruded into the core sheet layer.